1. Field of the Invention
The present invention relates to electroluminescence devices and more particularly to germanium electroluminescence devices operated by a Γ-valley transport across a heterojunction and fabricating methods of the same.
2. Description of the Related Art
An electroluminescence device is an optoelectronic device for converting electrical energy into light energy such as a light emitting diode (LED).
When a voltage is applied to both ends of a specific material layer (a light-emitting layer) of an electroluminescence device, electrons injected into a conduction band of a light-emitting layer are transited and recombined with holes in a valance band. At this time, by energy conservation, it emits a light corresponding to a transited energy bandgap (hereafter, simply called “a bandgap”).
To obtain transitions of electrons from a conduction band of a light-emitting layer to a valance band, momentum must be conserved before and after each transition.
On the other hand, energies of the conduction and valance bands are changed by the electron-moving direction in a lattice and are expressed to an energy (E)-wave number (k) diagram with a conduction band minimum and a valance band maximum
In an E-k diagram, a conduction band minimum and a valence band maximum are dependent on semiconductor materials and have the same k value or different k values. The former represents direct bandgap semiconductors and the latter represents indirect bandgap semiconductors.
In other words, conduction band minimums and valence band maximums of direct bandgap semiconductors (e.g., GaAs, GaN, etc.) exist at the Γ-point having k=0, while in indirect bandgap semiconductors (e.g., Si, Ge, etc.), valence band maximums exist at the Γ-point but conduction band minimums exist at the other points (points forming an X- or L-valley).
As the above mentioned, because momentum must be conserved before and after each transition to obtain transitions between the bands, the transited electrons have to get the same k value in the E-k diagram before and after each transition.
For this reason, electroluminescence devices have mainly formed a light-emitting layer with direct bandgap semiconductors until now.
By the way, the conventional electroluminescence devices using direct bandgap semiconductors have some problems. First, the manufacturing cost is too high to grow a light-emitting layer with direct bandgap compound semiconductors on a costly compound semiconductor substrate. Second, the conventional electroluminescence devices cannot be integrated together with common circuit elements generally fabricated on a silicon substrate.
Recently, studies on light-emitting layers based on silicon having an indirect bandgap or germanium enabling to easily grow on silicon are being actively progressed.
However, the studies until now are pursued with the objective of modifying the bandgap structure from the indirect bandgap to the direct bandgap by tensile and compressive strains through applying physical forces or injecting atoms with each other different size to silicon or germanium.
For example, U.S. Pat. No. 5,917,195 discloses a technology for a light emission by recombining between holes located at valence band maximum and electrons transported from an X-valley to a Γ-valley through changing the energy and momentum of electrons by lattice resonators such as phonon resonators in several resonating layers made with the radioactive isotopes of silicon Si28, Si29 and Si30. In this case, there are difficult processes to form several resonating layers using the radioactive isotopes of silicon Si28, Si29 and Si30.
And according to non-patent reference (Meng Liu et al, Band-Engineered Ge-on-Si Lasers, IEDM, pp. 146-149, 2010), a lattice structure of germanium of an indirect bandgap semiconductor is modified by a tensile strain for changing an energy band of conduction band, in other word, for regulating to raise the X-valley having a conduction band minimum and to come down the Γ-valley at k=0 to the similar energy of the X-valley, and then some electrons injected by doping of n-type impurities enable the transport into the Γ-valley for recombining with holes located at a valence band maximum. In this case, difficult processes to especially modify a lattice structure for forming a light-emitting layer with an indirect bandgap semiconductor are required.
Because the above mentioned conventional methods are commonly based on complex processes and are difficult to make mass productions possible, new electroluminescence device structures and fabricating methods of the same are needed to enable indirect bandgap material layer to be used as a light-emitting layer.
Specially, when a light-emitting layer is formed of germanium without modifying the bandgap, it generates a light with a wavelength 1550 nm which is little absorbed to a silicon oxide (silica) material of an optical communication. Accordingly, germanium can be used such as an infrared LED itself and a light source of an optical communication too. Thus, new germanium electroluminescence device structures and fabricating methods of the same are greatly needed.